The present invention relates to a method for production of a heat exchanger, which is brazed by using a powdery brazing filler metal, having excellent corrosion resistance and brazing property.
As a member of an assembled structure for a heat exchanger, a tube in which a brazing filler metal layer is formed on the surface of a thermally Zn-sprayed layer by coating a powdery brazing filler metal is already known (in patent publication No. Heisei 11-790). When this tube is brazed, the brazing filler metal will be molten and spread on the thermally Zn-sprayed surface causes Zn to uniformly diffuse on the surface of the tube. So, a uniform Zn-diffused layer will be formed on the surface of the tube, and the tube will have good corrosion resistance. If the tube is attached to a header and brazed with heating, it can be fixed.
On the other hand, a technique of coating a header with a powdery brazing filler metal is also known (in patent publication No. Heisei 10-175061). This technique allows the use of a bare material (extruded section) instead of a brazing sheet as the header, and the compressive strength of the header can be improved.
However, a heat exchanger, in which both the thermally Zn-sprayed tubes and the header are respectively provided with a brazing filler metal layer and assembled and brazed to each other, is not known.
Of the above-mentioned assembling methods, the former method is inferior since the bondability between the tubes and the header is not sufficient, depending on the brazing filler metal layer of the header. On the other hand, the latter method is superior since the tubes can be strongly brazed to the header. However, in the latter method, in the case where the thermally Zn-sprayed layer is formed on the surface of the tube, a layer having Zn at a high concentration is kept formed at the joint with the header in the longitudinal direction of the tube 10 as shown in FIG. 2, and corrosion 13 preferentially takes place in the layer, to pose a problem that it causes a through hole to be formed at the joint 12 between the tube 10 and the header 11.
The present invention has been completed in view of the above-mentioned situation. The object of this invention is to provide a method for production of a heat exchanger in which the tubes and the header are strongly bonded to each other and which has excellent corrosion resistance.
The method for production of a heat exchanger to solve the above-mentioned problem, as the subject matter of claim 1 comprises the steps of preparing a powdery brazing filler Al alloy consisting of 5 to 60 weight % of Si and the balance of Al and unavoidable impurities; classifying it into a fine alloy powder and a coarse alloy powder; depositing the fine alloy powder on the Al or Al alloy tubes having the thermally Zn-sprayed layer formed on the surfaces; depositing the coarse alloy powder on the Al or Al alloy header; and brazing the tubes and the header to each other.
The method for production of a heat exchanger as the subject matter of claim 2 conforms to claim 1, wherein the average particle size of the powdery brazing filler Al alloy is 0.5 to 200 xcexcm.
The method for production of the heat exchanger as the subject matter of claim 3 conforms to claim 2, wherein the average particle size of the powdery brazing filler metal of the brazing filler metal layer on the thermally Zn-sprayed tubes is 0.5 to 100 xcexcm, while the average particle size of the powdery brazing filler metal of the brazing filler metal layer on the header is 1 to 200 xcexcm, and the average particle size of the powdery brazing filler metal of the brazing filler metal layer on the header is larger than the average particle size of the powdery brazing filler metal of the brazing filler metal layer on the thermally Zn-sprayed tubes.
The method for production of the heat exchanger as the subject matter of claim 4 conforms to claim any one of claims 1 through 3, wherein the brazing filler metal layer of the thermally Zn-sprayed tubes and/or the brazing filler metal layer of the header contains a flux mixed.
The method for production of the heat exchanger as the subject matter of claim 5 conforms to claim 4, wherein the flux and the powdery brazing filler metal bond using a binder.
The method for production of the heat exchanger as the subject matter of claim 6 conforms to claims in any one of claims 1 through 3 or 5, wherein fins made of a Zn containing Al alloy dispose between the said tubes.
The method for production of the heat exchanger as the subject matter of claim 7 conforms to claim 4, wherein fins made of a Zn containing Al alloy dispose between the said tubes.
According to the heat exchanger produced by this invention, the brazing filler metal layer formed on the header allows the fillets of the brazing filler metal to be sufficiently formed at the joints with the tubes, and the Zn in the thermally Zn-sprayed layer diffuses quickly to average the Zn concentration. As a result, it is prevented that local corrosion takes place at the joints, to form through holes. Furthermore, the brazing filler metal layer formed on the thermally Zn-sprayed layer of the tubes contributes to the bonding with fins, and promotes the diffusion of Zn in the thermally Zn-sprayed layer, for uniforming the Zn concentration on the surfaces of the tubes as an action to enhance corrosion resistance. Moreover, Zn diffuses also into the brazing filler metal of the header, to improve the corrosion resistance at the joints.
The process for producing a heat exchanger as the subject matter of claim 1 can provide the above-mentioned action and effect, and since the prepared powdery brazing filler metal is classified for use, there is also an effect that the powdery brazing filler metal can be used without waste.
The present invention is described below in detail more.
[Si Content of a Powdery Brazing Filler Metal]
(In Common to Tubes and Header): 5 to 60 weight %
If the Si content is less than 5%, the function as a brazing filler metal is insufficient. If it is more than 60%, the erosion on the base metal becomes excessive to cause problems such as strength lowering, and furthermore, since the melting point of the brazing filler metal becomes 1150xc2x0 C. or higher, melting in the production of the powdery brazing filler metal becomes difficult. So, the Si content is limited in the afore-said range.
[Thermally Zn-sprayed Layer of Tubes]
The thermally Zn-sprayed layer of the tubes prevents that the Si of the brazing filler metal erodes the base metal, to form through holes, and hence corrosion resistance is also improved. It is preferred that the thermally sprayed amount of the thermally Zn-sprayed layer is in a range of 3 to 20 g/m2 (a further preferred range is 7 to 15 g/m2), for the reasons described below. If the thermally sprayed amount of Zn is less than 3 g/m2, the Zn concentration on the surface of the covering layer is too small to obtain a desired action. On the other hand, if the thermally sprayed amount of Zn is more than 20 g/m2, a large amount of Zn in the covering layer migrates into the fillets after brazing. So, the fillets are preferentially corroded causing the tubes to be disconnected from the fins, or to cause corrosion at the joints between the tubes and the header, or to raise the Zn concentration of the covering layer, remarkably increasing the corrosion rate.
[Formation of Brazing Filler Metal Layer on Tubes]
Since the brazing filler metal molten and spread on the thermally Zn-sprayed surface causes Zn to uniformly diffuse on the surfaces of the tubes, a uniform Zn-diffused layer can be formed on the surfaces of the tubes, to let the tubes have good corrosion resistance. Since the bonding with the fins and the tube can be achieved by means of the powdery brazing filler metal, a bare material can be used as the fins.
FIG. 3 shows the Zn concentrations on the surface of a brazed tube in the case where the tube has a thermally Zn-sprayed layer only without a brazing filler metal layer, and it shows that Zn concentrations are different from region to region.
FIG. 4 shows the Zn concentrations on the surface of a brazed tube in the case where the tube has brazing filler metal layer formed on a thermally Zn-sprayed layer, and it shows that Zn concentrations are uniform.
[Average Particle Size of Powdery Brazing Filler Metal]
(In the Brazing Filler Metal Layer of Tubes): 0.5 to 100 xcexcm
If the average particle size of the powdery brazing filler metal in the brazing filler metal layer of the tubes is less than 0.5 xcexcm, the powder is so fine that it becomes difficult to handle the powder, and since the total surface area of the powder as a whole becomes large, the amount of the oxide on the powder becomes so large as to lower the brazing property. If the average particle size of the powdery brazing filler metal is more than 100 xcexcm, the brazing filler metal cannot be applied thinly on the tubes, and the small clearance with the fins disposed between the tubes varies, making it hard to inhibit dimensional variations. Furthermore, the coated amount is likely to vary causing a problem that brazing failure is likely to occur. Therefore, it is desirable that the average particle size of the powdery brazing filler metal in the brazing filler metal layer of the tubes is in a range of 0.5 to 100 xcexcm as described in claim 2. It is more desirable that the lower limit is 1 xcexcm and that the upper limit is 50 xcexcm.
[Average Particle Size of Powdery Brazing Filler Metal]
(In Brazing Filler Metal Layer of Header): 1 to 200 xcexcm
If the average particle size of the brazing filler metal in the brazing filler metal layer of the header is less than 1 xcexcm, the powder is so fine that it becomes difficult to handle the powder, and since the total surface area of the powder as a whole becomes large, the amount of the oxide on the powder becomes so large as to lower the brazing property. Since especially the header is larger in wall thickness than the tubes, when the brazing, the header is harder to raise the temperature than the tubes, and the average particle size greatly affects the brazing property. If the brazing property of the header declines, the bonding strength with the tubes declines, and furthermore Zn is not diffused sufficiently, being liable to cause local corrosion. As for the upper limit of the average particle size of the brazing filler metal in the brazing filler metal layer of the header, since there is no limit to the size unlike the tubes, a larger amount can be applied than that for the tubes. So, the upper limit can be 200 xcexcm. Therefore, it is desirable that the average particle size of the powdery brazing filler metal in the brazing filler layer of the header is in a range of 1 to 200 xcexcm as described in claim 2. It is more desirable that the lower limit is 51 xcexcm and that the upper limit is 150 xcexcm.
[Average Particle Size Ratio of Powdery Brazing Filler Metal]
(Average Particle Size of Brazing Filler Metal Layer of Header greater than Average Particle Size of Brazing Filler Metal Layer of Tubes)
It is desirable that the particle size of the powdery brazing filler metal of the tubes is smaller within the afore-said range and that the particle size of the powdery brazing filler metal of the header is larger within said range, as described above. If the average particle size of the powdery brazing filler metal in the brazing filler metal layer of the header is equal to or smaller than that of the powdery brazing filler metal in the brazing filler metal layer of the tubes, the melting of the brazing filler metal on the header side delays, and the brazing filler metal on the tube side is molten locally at the joints, to restrict the diffusion area of Zn. Depending on the brazing filler metal on the header side that is molten late, Zn is not sufficiently diffused, and a layer locally high in Zn concentration remains. Therefore, it is desirable that the average particle size ratio of both is such that the average particle size of the brazing filler metal layer of the header is larger than that of the brazing filler metal layer of the tubes as described in claim 2.
[Classification of Powdery Brazing Filler Metal]
For classifying the powdery brazing filler metal, various methods can be used, and such a method as sieving, air classification or wet classification can be used. For classification, a powdery brazing filler metal with an average particle size of 0.5 to 200 xcexcm can be used. Furthermore, for classification, one reference size can be set for classifying the powder into a fine powder and a coarse powder in reference to the reference size. In another aspect, one reference size for a fine powder and another reference size for a coarse powder can be set for classifying the powder.
In view of efficiency, it is desirable to classify in reference to one reference size. The reference size of classification can be 32 to 100 xcexcm.